In order to have an instrument which has the test chamber area distanced from the measurement area, two types of extensometers have been developed. These are vertical extensometers and horizontal extensometers. An example of a vertical extensometer is shown in my U.S. Pat. No. 4,848,161, wherein the extensometer is offset to one side of the test specimen, allowing the test specimen strain to be transferred by the extensometer, to its measurement portion, below the test chamber (furnace).
The present invention relates to horizontal extensometers wherein the extensometer arms or clamping mechanism for the specimen are horizontally oriented and the test specimen is placed into the test chamber in a vertical orientation.
Known in the present art are two methods for attachment to the test specimens in horizontal extensometers. The first is to press hinged extension arms against one side of the test specimen. The second is to hold the specimens with offset arms or offset telescoping arms which clamp the specimen by means of a remote spring. The offset arms are combined with a flexural hinge which may, or may not, be part of a transducer.
An example of the first method is found in U.K. patent application published Sep. 7, 1988, under No. GB2201797A. In this particular instrument, hinged extension arms press against the test specimen. Unfortunately, the instrument suffers from a serious disadvantage. It places a bending moment on the test specimen which is undesirable and becomes more undesirable as test temperatures increase.
An example of the second type of extensometer is found in U.S. application Ser. No. 895,642 by John S. Korellis for a CLIP-ON EXTENSOMETER GRIP. This device requires an extensometer with offset arms having significant mass, and inertia, which must be overcome by increased spring force to urge knife edges against the specimen in dynamic testing. The flexural hinge requires an activation force which must be supplied by the tensile test specimen. Again, because of the extreme weight of the instrument and the forces required to be supplied by, and imparted to the tensile specimen, accuracy of measurement is sacrificed.
More recently, it has been found that to overcome the problems caused by the excess forces and weights present in extensometers with offset arms, a linear inline clamping of the test specimen can be used. A spring clamping force is applied so that it creates a very low disturbing moment on the specimen. The clamping force is applied directly diametrically across from the specimen engaging edge of the extensometer.
This type of device is shown in published European patent application No. 84,302,636.0. One of the problems with the clamping mechanism of this device, is that the unitary spring or clip is located within the test chamber and thus subject to the test environment. As such, the clamping force of the spring or clip is reduced with increasing temperature, thus, accurate knife edge location is compromised. Again, accuracy of measurement is sacrificed.
It is an object of the present invention to provide a novel horizontal extensometer which overcomes the measurement accuracy problems of the prior art and which accurately transfers motion to the transducer, with near zero hysteresis.